The present invention relates to a method and system for removing debris from a well where the debris has accumulated both above the downhole pump and at the downhole pump intake. In particular, it relates to a downhole tool that isolates the perforations and allows the debris to be circulated to surface without requiring a special tubing string to be run from surface.
One application for this invention is in the production of heavy oil from subsurface reservoirs where current operating practices encourage the co-production of reservoir sand with reservoir fluids. This sand can accumulate in the well annulus to impair flow to the downhole pump intake and accumulate in the production tubing above the pump, thereby impairing, and in many cases, halting pump operation. Consequently, sand clean-outs typically account for more than half of well servicing activity in heavy oil operations.
Conventional well servicing techniques require all of the downhole equipment to be pulled from the well prior to running in a sand bailer of some design. This requires a workover rig and often several days of bailing to get the well clean enough to re-install the downhole pump. This invention describes a new method and system that allows sanded in downhole pumps to be restarted without pulling them from the well.
Another method that is currently used for cleaning out wells is to load the well annulus with fluid from surface, which flushes the sand from the wellbore back into the formation through the perforations. This may effectively remove sand from the wellbore near the top of the perforated interval, but since the pump intake is typically located below the perforations, loading the annulus will likely not mobilize any sand accumulated around the pump intake.
This method also does not mobilize sand accumulations in the production tubing above the downhole pump.
For downhole pumps driven by sucker rods, sand accumulated above the pump can seize the rod string in the production tubing, thereby preventing the pump from being restarted even if the pump intake is clear. The debris accumulated in the tubing must be removed to allow the pump to be restarted. This debris could be mobilized and circulated to surface by injecting fluid into the production tubing just above the downhole pump. It is, however, not practical to install an injection tubing string from surface for this purpose due to the expense of the tubing and the added rig time and aggravation caused by strapping the injection tubing to the production tubing during installation of the downhole equipment.
This invention describes a new method and system that allows downhole pumps that are clogged with sand or other debris, here generally referred to as xe2x80x9cdebrisxe2x80x9d, to be restarted without pulling them from the well.
According to one aspect of the invention, the invention provides a method based on circulating debris out of the wellbore using the installed downhole pump instead of removing the downhole pump and bailing debris from the well. This is accomplished by mobilizing the debris in the production tubing above the downhole pump and at the downhole pump intake, then restarting it. Debris is mobilized by introducing fluid into the debris pack in both the well annulus and the production tubing above the downhole pump.
In another aspect of the invention, a tube is inserted near the downhole pump intake so that fluid can be pumped directly to the pump intake so that the debris can be mobilized.
In another aspect of the invention, there is provided a system that allows fluid to be circulated down the well annulus, through a port in the production tubing immediately above the downhole pump and back to surface. This allows debris to be circulated out of the well to surface prior to restarting the pump. The port in the production tubing, is configured with a check valve to prevent fluid loss during normal pumping operations.
To circulate the debris accumulated in the production tubing to surface may require a downhole pressure applied to the annulus that is significantly higher than the reservoir pressure. Therefore, to prevent the injected fluid from entering the reservoir, the perforations must be isolated from the well annulus above the downhole pump. It is thus proposed in accordance with another aspect of the invention, that a flexible sealing element be installed, preferably approximately one joint above the downhole pump.
Therefore, in accordance with an aspect of the invention, there is provided an apparatus for cleaning debris from wells, the apparatus comprising a tubular tool body having connection ends for installation in a production tubing string, the tubular tool body having an exterior and interior and one end of the tubular tool body being an upper end; and a flexible annulus sealing element disposed around the tubular tool body.
In accordance with another aspect of the invention, the flexible annulus seal allows the upward flow of formation gas during normal pump operation but forms a seal in the annulus when fluid is pumped into the well annulus from surface.
In accordance with another aspect of the invention, there is provided at least one bypass tubing defining a flow passageway from the exterior of the tubular tool body above the flexible annulus sealing element to the interior of the tubular tool body at some location or locations above the downhole pump.
In accordance with another aspect of the invention, there is provided at least one bypass tubing defining a flow passageway from the exterior of the tubular tool body above the flexible annulus sealing element to the exterior of the tubular tool body below the flexible annulus sealing element and continuing to near the pump intake.
In accordance with another aspect of the invention, each bypass tubing may be configured with a check valve to control flow during different stages of the well cleanout operation. The check valve may be pressure sensitive so that it closes at one threshold pressure and opens again once the pressure falls below that threshold. The threshold pressure, where the check valve closes, would be selected to correspond to slightly above the hydrostatic pressure that occurs when the well annulus is filled with the work over fluid.
In accordance with another aspect of the invention, there may be provided centralizers disposed around the tubular tool body to protect the flexible annulus sealing element.
In accordance with a further aspect of the invention, there is provided a method of cleaning debris from a wellbore, in which the wellbore contains production tubing and a pump having a pump intake, the wellbore and exterior of the production tubing defining an annulus, the method comprising the steps of: sealing the annulus above the downhole pump by pumping fluid into the wellbore annulus from surface to activate a flexible annulus sealing element to isolate the producing reservoir; and conducting the fluid from the annulus above the flexible annulus sealing element to a location in which debris has accumulated, as for example, adjacent the downhole pump intake or within the production tubing above the downhole pump, to thereby mobilize the debris and permit the debris to be removed.
In a further aspect of the method of the invention, there is provided the step of continuing to pump fluid from surface into the wellbore annulus to circulate the debris back to surface through the production tubing.
In a further aspect of the method of the invention, there is provided the step of starting the downhole pump to aid in circulating the debris to surface and to resume production of fluids from the reservoir.
In a further aspect of the method of the invention, there is provided the step of allowing the fluid in the well annulus above the flexible annulus sealing element to drain at a controlled rate to the pump intake to dilute further influxes of debris.
In a further aspect of the method of the invention, there is provided the step of relaxing the flexible annulus sealing element to allow the upward flow of formation gas in the well annulus during normal pump operation.